Preheat the oven to 350°F (175°C) and grease a 9x13-inch pan.
In a bowl, whisk together flour, baking powder, baking soda, cinnamon, and salt.
In another bowl, mix sugars, oil, eggs, and vanilla until smooth.
Gradually add dry ingredients to wet ingredients, mixing gently.
Fold in grated carrots until evenly combined.
Pour batter into the prepared pan and smooth the top.
Bake for 35–40 minutes, until a toothpick comes out clean.
Let the cake cool completely.
Beat frosting ingredients until smooth and spread over cooled cake.